Pool-type discharge device



y 1941- K. H. KINGDON ETAL 2,242,736

POOL-TYPE DISCHARGE DEVICE Filed March 50, 1939 ,2 Sheets-'Sheet 1 51 aF153. I. 24

Inventors: Kenneth H. Kingdom Elliott J. Lawton,

' Them Attorney.

y 20, 19 1- K. H. KINGDbN HAL 2,242,786

POOL-TYPE DISCHARGE DEVICE Filed March 30, 1959 2 Sheets-Sheet 2IIIIIIIIIIIIIIIIIIIIIIIIIIIII/fl We Inventors: Y

e Kenneth H. Kin don, W El|iottJ.Law%on,-

/vw ajwifl.

Them Attorney.

Patented May 20, 1941 U lTE POOL-TYPE DISCHARGE DEVICE Application March30, 1939, Serial No. 264,950

8 Claims. (Cl. 250-275) The present invention relates to improved meansfor initiating a discharge between the principal electrodes of apool-type discharge device, such as a mercury rectifier. While notlimited thereto, the invention is primarily applicable to single anodedevices, in which the main discharge requires to be reinitiated atcyclically recurrent intervals.

In common with certain known systems our invention utilizes acontinuously maintained with further objects and advantages thereof, may

best be understood by reference to the following description taken inconnection with the drawings in which Fig. 1 shows in longitudinalsection a discharge device suitably embodying the invention; Fig. 2 isan enlarged detail view of a fragmentary portion of the device of Fig.

1; Fig. 3 is an incomplete sectional view taken on line 33 of Fig. 1;Fig. 4 is a diagrammatic representation useful in explaining theoperation of the invention; Fig. 5 shows one possible modification ofthe invention; and Figs. 6 and 7 are different sectional views of asecond modification.

Referring particularly to Fig. 1 there is shown an elongated cylindricalenvelope comprising two spaced metal cylinders Hi and II. These form achamber or jacket for receiving a cooling fluid which is introduced andwithdrawn by means of inlet and outlet connections I4 and 1'5.

The envelope is closed at its ends by means of transversely extendingcircular headers I7 and I8 which are peripherally joined to the wallmember H as by being welded thereto. At the upper end of the envelopethere is provided an anode 20 which is insulatingly supported from theheader l'l by means of a glass-to-metal seal. The glass element of theseal comprises a ring 2| which is joined at its extremities tocooperating metal parts 22 and 23. The points of conjunction of theglass and metal are protected against the destructive eifects of thedischarge by means of shielding members 24 and 25. A

heavy conducting rod 26 serves both to support the anode and to connectit with an external circuit which includes a load device, illustrated asa resistor 28.

For cooperating with the anode 20 in conducting a discharge through theenvelope there is provided a pool-type cathode 27 consisting, forexample, of a body of mercury. This is in direct contact with the headerl8 and therefore is electrically continuous with the whole body of theenvelope. It may be connected to a source of potential 50 by means of aheavy conducting stud 29 which is affixed directly to the bottom of theheader l8.

Assuming that the device illustrated is intended as a rectifier, it mustfunction to conduct a cyclically interrupted discharge between the anode2B and the cathode 27. In order to make possible the reinitiation of thedischarge after each non-conducting half-cycle, there is provided meansfor maintaining a holding-arc to the surface of the cathode. While suchmeans is shown as comprising a single anode 3i], energized by connectionto a direct current source ,32, it may alternatively comprise twoelectrodes connected to a source of alternating potential. The magnitudeof the arc-current is limited by the provision of a series resistor 33.

The holding-arc is ignited during initial starting of the dischargedevice by means of an immersion starting electrode as shown at 34. Inaccordance with known practice, this electrode may consist of a body ofsemi-conducting material having its tip portion maintained in contactwith the cathode 27. It may be energized by being connected through aswitch 35 to' an appropriate terminal of the battery 32.

It is desirable to provide means to prevent the cathode spot of theholding are from becoming attached to the metal wall of the envelope. Tothis end there is provided a housing structure which substantiallyencloses the discharge path for the holding-arc and segregates it fromthe main discharge space. By this means the oathode spot of the holdingarc is prevented from Wandering freely over the cathode surface, and atthe same time the holding arc is effectively shielded from extinction byevents taking place in the main discharge space.

The particular housing construction which is shown in Fig. 1 comprises acylindrical member 38 which extends from a point below the cathodesurface up to and around the holding anode 30. The housing is closed atthe top by means of a circular plug consisting, for example, of aninsulating material such as lava. A restricted opening in the form of anarrow slot 49 extending normally to the cathode surface providessubstantially the only communication between the interior of the housing38 and the main discharge space. This slot may be on the order of a fewmillimeters in width. For reasons which will be explained more fully inthe following, it is desirable that at least the portion of the housingsurrounding the opening 49 be constituted of insulating material. Thisbeing so, it is convenient to make the entire housing of a refractoryinsulator, for example, quartz.

We are aware that it is known to use a shielding housing for theprotection of the holding arc. In the prior art arrangements with whichwe are familiar, however, successful operation depends upon having thecathode spot of the holding arc relatively accessible to the outsidespace so as to permit the ready ignition of the main dischargetherefrom. For example, in the shielded holding-arc arrangement which isdescribed and claimed in Patent No. 2,152,201, granted March 28, 1939,to L. D. Miles, the cathode spot for the main anode is formed bysub-division of the cathode spot for the holding are. In such anarrangement there is some tendency for the latter cathode spot to bedrawn outside the shielding enclosure and, under unfavorable conditions,extinguished. Our present invention is distinguished by the inclusion ofmeans by which the cathode spot for the holding arc may be retained atall times at a point which is well shielded from the main dischargespace. In the present case, for example, we prefer to use an anchoringbody t2 which may consist of a clean molybdenum rod projecting slightlyabove the surface of the mercury so as to provide an anchorage for theholding arc cathode spot at a point which is .relatively displaced fromthe slot 40.

In order to assure the ready ignition of the main discharge in spite ofthe confinement of the holding arc cathode spot, our invention providesmeans for causing the mercury to wet at least the portion of theinsulating housing 38 which surrounds the opening 46. This expedient,which results in a reversal of the mercury meniscus, is efiective tocause striking of a separate cathode spot for themain discharge at theline of intersection of the mercury and the surface of the housing. Thespot, once started, is free to spread as required to supply the currentdemands of the main discharge.

In order to obtain the result indicated in the foregoing, the portion ofthe housing 33 which surrounds the opening 40 is made of insulatingmaterial and is provided with a coating of a finely divided metal whichis capable of being wet by mercury but which is not appreciably attackedby it. Numerous materials may serve in this connection, and molybdenum,iron and tungsten may be mentioned .as examples. The first of these isconsidered preferable.

The arrangement referred to above is illustrated in greater detail inFig. 2 which shows an enlarged section of the slot 40. The shading ofthe surface of the housing in the vicinity of the slot is intended todesignate a finely divided metallic deposit. It will be seen that as aresult of the presence of this deposit the mercury tends to creep up thehousing wall so as to form a tapered contact therewith. It has beenfound that a contact of this nature is very helpful in making possiblethe ready initiation of a cathode spot at the line of intersection ofthe mercury holding-arc cathode spot.

and the housing. While we do not wish to be bound to any particularexplanation of this phenomenon, we consider that it is due to the factthat the metallic coating provided on the housing results in theexistence of a surface area in which small conducting elements areinterspersed with small insulating elements. Under these conditions theaccumulation of charges on the insulating surfaces may produce extremelyhigh potential gradients sufiicient to initiate a discharge by themechanism of field emission. If such a discharge is once initiated at apoint close to the mercury surface, it may readily spread to suchsurface, as previously sug- .gested.

A metallic coating of the desired character may advantageously consistof metal which has been sputtered on the surface of the housing. It isfound, however, that unless such a coating is maintained in somefashion, it tends to be automatically removed after a brief period ofdischarge operation. In order that this may not occur, our inventionfurther provides means for restoring the sputtered coating of metal. Inthe arrangement shown such means comprises a metallic member in thenature of 'a probe electrode '45 positioned in the opening do. Thiselectrode, which preferably is constituted of a molybdenum wire, may beconnected with an external source of potential through a leadinconductor 6 which comprises a continuous extension of the electrodeitself. In order to protect this conductor from the effects of the "aredischarge it is covered with a suitable insulating sheath 41 consistingfor example of glass or quartz. By virtue of the arrangement indicatedadditional material may be sputtered from the surface of the electrode45 as required to maintain .a metallic coating on the surface of thehousing 38.

The electrode 45 serves a further function in the operation of theapparatus in assisting the cyclical initiation of the main dischargefrom the holding-arc. In order to explain this feature a full cycle ofstarting operation will be described.

With the holding-arc to the anode 3B in operation, a positive potentialimpressed on the electrode 45 will initiate a glow discharge from thiselectrode to the surface of the cathode 21. Because of the ionizationprovided in the opening 40 by this discharge, the main anode 20 isenabled to pick up current from the anchored This current flows throughthe slot 49 and exposes the walls of the housing in the vicinity of theslot to intense ionization. When the current drawn by the main anodehas-reached an appreciable value, say several amperes, a new cathodespot :will be struck adjacent to the line of intersection of the mercuryand the housing, as previously described. This new cathode spot is freeto move over the surface of the cathode outside the housing 38. Theholding-arc spot, on the other hand, remains anchored at 42 throughoutthe entire starting period.

In order to assure fulfillment of both the functions described in theforegoing, it is desirable that the electrode 45 be energized in anappropriate fashion. To this end, if it be assumed that the maindischarge receives its energy from the first phase 50 of a three-phasealternating current source, then the electrode 45 .may suitably beenergized from asecond phase 5|, which is a leading phase with respectto the phase 53. Under these circumstances the cycle of operation willbe as indicated in the diagrammatic representation of Fig. 4. In thisfigure, the curve A is assumed to represent the potential applied to theelectrode 45, while the curve B represents the main anode potential. Innormal operation a glow discharge to the electrode 85 will be initiatedat the point a or shortly thereafter. This discharge will continue tothe point D, at which time the potential of the electrode becomesnegative. At an intermediate point the anode 26 will become positive andstart to draw current and at a fourth point, say 12, a new cathode spotfor the main discharge will be struck. Normal conduction will then occurthroughout the remainder of the half-cycle. It will be understood alsothat the time of starting of the main discharge may be to a certainextent controlled by controlling the phase relationship of the potentialapplied to the electrode 45. We therefore consider the use of theelectrode 45 as a control element to be within the scope of ourinvention. To this end the current-limiting resistor 52 may be replacedby appropriate phase-shifting means.

As a result of the fact that the electrode 45 is at a negative potentialduring an appreciable portion of the main discharge cycle, that is tosay, between the points I) and 6, it will be sub- J'ected to substantialpositive ion bombardment during this period. The sputtering which thusresults serves to renew the metallic coating on the surface of thehousing and to assure continued operation of the device in the desiredmanner.

Since the arrangement described permits the cathode spot of the holdingarc to be maintained at all times at a point relatively remote from theslot 49, it is effectively shielded from the elfects of occurrences inthe main discharge space. Consequently there is little likelihood thatthe holding-arc will be extinguished at any time, and the operation ofthe apparatus as a whole is rendered highly reliable.

An alternative arrangement which results in still more completeshielding of the holding arc is shown in Fig. of the drawings, whichillustrates only the holding arc assembly. In this case the holding arcis maintained to the cathode 59 by means of a pair of cooperatingkeep-alive electrodes 69 and 6| which are separated by means of asuitable balile 82. Both keep-alive electrodes are enclosed in aninverted cuplike housing 6 consisting of quartz or the like.Substantially the only communication between the interior of the housingand the main discharge space is provided through a tubulation 65 whichextends laterally from the side of the housing. The extremity of thistubulation is positioned beneath the surface of the mercury, but it isprovided with a slot 61 which opens to the discharge space in a mannersimilar to the slot All of Fig. 1. There is also provided in connectionwith the slot 61 a sputterable electrode 68 which corresponds in natureand function with the electrode 45 previously described. The mechanismof discharge initiation is precisely the same in the arrangement of Fig.5 as in that of Fig. 1.

Figs. 6 and 7 illustrate a still further variation of the invention inwhich magnetic means are provided for retaining the holding-arc cathodespot at a point which is thoroughly shielded from the main discharge. Inthis case there is provided in connection with the holding anode "H1 anelongated insulating housing H, which is provided at one end with anarrow slot 72. Outside the housing and in a region between the anodeIll and slot 1 2 there is mounted a magnet is having its respectivepoles disposed on opposite sides of the housing. This magnet produces a.field transverse to the axis of the housing and thus serves to preventthe cathode spot associated with the holding arm from approaching theslot '12. (The use of a magnetic field in confining the motion of acathode spot on the surface of a pool-type cathode is fully describedand claimed in L. D. Miles application, Serial No, 254,156, filedFebruary 2, 1939.)

In connection with the slot 12 there is provided a metallic electrode 76which serves to maintain a sputtered coating on the wall of the housingadjacent the slot and also to facilitate the picking up of themaindischarge. Except for the structural differences emphasized in theforegoing, the arrangement of Figs. 6 and '7 is similar in function tothat of Fig. 1.

'While we have described our invention in connection with particularembodiments thereof, it will be understood that numerous modificationsmay be made by those skilled in the art. We, therefore, aim in theappended claims to cover all such equivalent variations as come withinthe true spirit and scope of the foregoing disclosure.

What we claim as new and desire to secure by Letters Patent of theUnited States, is:

1. An electric discharge device comprising a main anode, a mercury poolcathode, means for maintaining a holding-arc having a cathode spot onthe mercury pool surface, a housing substantially enclosing thedischarge path for the holding-arc except for a restricted opening nearthe cathode surface, at least a portion of the housing surrounding theopening being constituted of insulating material, and a coating offinely divided metal in mutual contact with the said in-, sulatingmaterial and with the cathode at its surface, the said metal serving inconjunction with the said insulating material to facilitate theestablishment of a separate cathode spot for the main anode upon theapplication to the anode of a discharge-favoring potential.

2. An electric discharge device comprising a main anode, a pool-typecathode, an auxiliary anode for maintaining a holding arc having acathode spot on the cathode surface, a housing enclosing the dischargepath for the holding arc except for a restricted opening at the cathodesurface, the portion of said housing surrounding the opening beingconstituted of insulating material, and means for maintaining a thincoating of finely divided metal on the surface of the said portion ofthe housing, thereby to facilitate the initiation of a cathode spot forthe main anode in the vicinity of the opening.

3. An electric discharge device comprising a main anode, a pool-typecathode, an auxiliary anode for maintaining a. holding-arc having acathode spot on the surface of the cathode, a

housing enclosing the discharge path for the holding-arc except for arestricted opening at the cathode surface, the portion of said housingsurrounding the opening being constituted of insulating material, acoating of finely divided metal applied to the said portion of thehousing for facilitating the establishment of a separate cathode spotfor the main anode upon the occurrence of ionization in the vicinity ofthe coated region, and a metallic electrode in the vicinity of the saidopening and assisting the initiation of an ionization-producingdischarge to the main anode from the holding-arc cathode spot.

4. An electric discharge device comprising a main anode, a pool-typecathode, an auxiliary anode for maintaining a holding arc to the oathodesurface, a housing enclosing the discharge path for the holding arcexcept for a restricted opening at the cathode surface, a portion ofsaid housing surrounding the opening being constituted of insulatingmaterial, and a metallic electrode positioned in the vicinity of theopening, said electrode serving the dual purpose of assisting theinitiation of a discharge to the main anode from the cathode spotassociated with the holding-arc and of maintaining a coating ofsputtered metal on the said portion of the housing, thereby tofacilitate the formation at the coated region of a separate cathode spotfor the discharge to the main anode.

5. An electric discharge device comprising a main anode, a pool-typecathode, an auxiliary anode for maintaining a holding-arc to the oathodesurface, a housing enclosing the discharge path for the holding-arcexcept for a restricted opening at the cathode surface, a portion ofsaid housing surrounding the opening being constituted of insulatingmaterial, and an electrode body consisting of molybdenum positioned inthe vicinity of the opening, said electrode body serving the dualpurpose of assisting the initiation of a discharge to the main anodefrom the oathode spot associated with the holding arc and of maintaininga coating of sputtered molybdenum on the said portion of the housing,thereby to facilitate the formation at the coated region of a separatecathode spot for the discharge to the main anode.

6. An electric discharge device comprising a main anode, pool-typecathode, an auxiliary anode for maintaining a holding arc to the cathodesurface, a housing enclosing the discharge path for the holding-arcexcept for a restricted opening at the cathode surface, at least aportion of said housing surrounding the opening being constituted ofinsulating material, means for confining the cathode spot of theholding-arc to a region of the cathode surface relatively remote fromthe said opening and means for maintaining a coating of sputtered metalon the surface of the said portion of the housing, thereby to facilitatethe initiation of a cathode spot for the main anode in the vicinity ofthe opening.

'7. An electric discharge device comprising a main anode, a pool-typecathode, an auxiliary anode for maintaining a holding-arc having acathode spot on the surface of the cathode, a housing enclosing thedischarge path for the holding-arc except for a restricted opening atthe cathode surface, a portion of said housing surrounding the openingbeing constituted of insulating material, means including a metallicelectrode positioned in the vicinity of the opening for maintaining acoating of sputtered metal on the said portion of the housing, therebyto facilitate the establishment of a separate cathode spot for the mainanode upon the occurrence of ionization in the vicinity of the coatedregion and lead-in connections for the said electrode, whereby thepotential of the electrode may be varied in such fashion as to controlthe initiation of an ionization-producing discharge to the main anode.

8. An electron discharge device comprising a main anode, a mercury poolcathode, means for maintaining a holding-arc having a cathode spot onthe surface of the mercury pool, a housing substantially enclosing thedischarge path for the holding-arc except for a restricted opening atthe cathode surface, means providing insulating surfaces continuous withthe cathode in proximity to the said opening, and means applied to saidsurfaces for reversing the normal meniscus of the mercury to bring thesame into tapering contact with the surfaces, whereby ionizationproduced in the vicinity of the said surfaces by current drawn from theholding-arc cathode spot to the main anode may readily establish aseparate cathode spot for the main anode at the region of reversedmeniscus.

KENNETH H. KINGDON. ELLIOTT J. LAWTON.

